This invention relates to screen-printable electroluminescent (EL) thick film compositions and their use in the formation of EL panels or lamps. The terms “panel” and “lamp” are meant to be equivalent.
An EL lamp or panel is typically constructed using ITO (Indium Tin Oxide) sputtered polyester (PET) as the base substrate. Typical thicknesses of the ITO-sputtered PET are 5.0 to 7.0 mils. The actual ITO thicknesses range from about 750-1500 nm. A phosphor-containing layer is then screen-printed on to the ITO-sputtered polyester and then dried at approximately 130° C. for 5-10 minutes in either a box oven or a belt-driven oven. The phosphor used is typically a transition metal-doped ZnS. Next, a dielectric layer is screen-printed and dried as above. Usually, two dielectric layers are deposited and dried. Finally, a rear electrode conductor layer is printed and dried as above to form the completed lamp. As constructed, an EL lamp is basically a capacitor, with the ITO layer functioning as a clear conductive layer which enables the light emitted by the phosphor layer to pass through the front of the lamp with little or no attenuation. These lamps typically run on AC (alternating current).
The screen-printable inks or pastes used to fabricate an EL lamp are composed of a resin or polymer dissolved in a solvent, and a functional component which helps impart the particular functionality for a given composition. The resins used in the vast majority of current EL lamps belong to the general class known as fluoropolymers. Other resins such as cyanoethylated starches or epoxies have also been employed. Unfortunately, due to solubility problems, the preferred glycol ethers cannot typically be used. As noted above, the phosphor paste contains ZnS particles suspended in the resin/solvent solution, the latter to be referred to as a medium. The dielectric or insulating layer paste contains a high dielectric constant material such as Barium Titanate powder dispersed in a fluoropolymer-based medium.
The rear electrode silver paste is composed of silver flake dispersed in a medium as defined above, while a carbon electrode paste uses conductive carbon black and graphite dispersed in the appropriate medium. The use of carbon as a rear electrode is more prevalent due to the lower cost of the material and the tendency for silver to migrate in the presence of water when the EL lamp is placed under bias.
Carbon electrodes may be substituted for silver electrodes without much sacrifice in performance when constructing small EL lamps (eg. 2×2 inches). For larger lamps, the use of a silver electrode would be desirable in that the lower-resistivity silver leads to a more uniformly-lit EL lamp.
The following patents demonstrate the state of the prior art.
U.S. Pat. No. 6,445,128 to Bush et. al. discloses an EL panel made with PVDF/HFP copolymer resin binder, in substantially an uncrosslinked form, with DMAC solvent and/or other higher boiling point solvents/latent solvents/extenders. The resin binder is characterized by a melt viscosity of 1.0-8.5 kP and a Differential Scanning Calorimeter (DSC) melt temperature of 103-115° C.
Since EL lamps degrade exponentially over time, it is a key concern to find new ways to decrease this degradation thus improving lamp lifetime. The rate of degradation is related to the voltage/frequency the lamp is run at, and the temperature/humidity the lamp is exposed to. The higher the voltage/frequency, the worse the degradation is. Similarly, the higher the temperature/humidity is, the worse the degradation. A desirable improvement in EL lamps would then be to extend the usable lifetime. That is, since EL lamps are usually monitored according to the percentage of their original brightness, a lamp that has 50% of its initial brightness after 200 hours is more useful than a lamp that has 50% of its initial brightness after 100 hrs. Extending the lifetime would open new markets for EL lamps. Facilitating the use of the polymers by making them more soluble in common solvents such as glycol ethers would also be very desirable.
In view of the above, it is therefore the object of this invention to provide screen-printable pastes for use in fabricating EL lamps which exhibit improved or extended lifetimes.
Another object of this invention is to produce brighter EL lamps using silver as the rear electrode.
Further, an additional object of this invention is to produce screen-printable pastes for EL lamps that utilize silver as a rear electrode and do not exhibit significant silver migration.
A further object of this invention is to provide screen-printable pastes for EL lamps that contain a co-polymer of PVDF/HFP as an integral part of the medium of at least one such paste in the lamp construction.
Yet another object of this invention is to provide screen-printable pastes for EL lamps that have stable solution viscosities and do not lead to increased incidences of “short circuits” as the solution ages and are soluble in glycol ether solvents.
We have unexpectedly found that significant advantages can be observed with PVDF/HFP copolymers when both the melt temperature and melt viscosities are significantly different from the ranges of values noted in the prior art. It has been found that the solution viscosities of the PVDF/HFP copolymers used in the present invention are much more stable over time than those defined in the prior art. This is a significant improvement in that the tendency to have “short circuits” occur over time is drastically reduced using the PVDF/HFP copolymers noted herein as compared with those in the prior art.